Active components of electronic systems, i.e. integrated circuits (ICs), such as microprocessors, controllers, ASICS (application specific ICs) and other microelectronic devices, are a source of unwanted radiated electromagnetic emissions which may cause electromagnetic interference (EMI). To prevent interference, radiated emissions from equipment must be controlled to meet various regulatory requirements and standards. Control of undesirable electromagnetic emissions may add significant cost to an electronic system. Suppression of radiated emissions may be accomplished at the system level, by providing a shielded enclosure for an apparatus, or by enclosing parts of the apparatus, e.g. a circuit board, in a shielded housing. In an example described in U.S. Pat. No. 4,661,888 to Jewell, entitled "Removable modular housing for RF circuits" issued 28 Apr. 87, a circuit board is enclosed in a grounded metal, i.e. aluminium, casing. However, in many applications grounded metal enclosure or housing of this type is bulky, uneconomical and impractical for maintenance purposes.
As described in U.S. Pat. No. 4,459,680 to Sukeda issued July 1984, entitled "Magnetic Bubble memory cassette" EMI shielding is provided in the form of a ferromagnetic enclosure. Sukeda shows a packaged integrated circuit with associated components mounted on a printed circuit board (PCB), with a shield plate, comprising a sheet of iron or permalloy (0.5 to 0.3 mm thick) which surrounds a fully operational device and the substrate supporting it, that is, a circuit board and connectors. Use of electrically conductive shielding materials, including ferromagnetic metals and alloys, which. rely on an electro-conductive effect for their shielding, requires that the grounded shielding layer is sufficiently spaced from other conductors, such as terminal pins of the enclosed IC package and the conductive traces on the PCB, to prevent unwanted electrically conductive paths or shorts. Thus, the shielding plates may substantially increase the size of the packaged component, and may increase thermal stress by reducing heat dissipation.
Conventional packages for integrated circuits are formed from dielectric materials, typically, plastic or ceramic materials, which provide physical and environmental protection of the enclosed chip, but provide little or no attenuation of electromagnetic emission generated during operation of the enclosed circuit. Thus, improved circuit design and layout is used to achieve suppression at the source of radiation, e.g. from the integrated circuit itself. However, this solution may add to circuit design complexity and cost, or compromise performance.
Alternatively, additional components on a circuit board, such as filters, chokes, decoupling capacitors, etc., are required to reduce or control electromagnetic emissions.